Find the element at a given position in a sorted version of an unsorted array. Example: Input: {5,1,7,19,0,16}, k=3 Output: 5 (Explanation: Sorted array is {0,1,5,7,16,19}, and the 3rd positioned element is 5.)

To find the k-th smallest element in an unsorted array, the most efficient approach is to use the Quickselect algorithm. Quickselect is a selection algorithm that finds the k-th smallest element in an unordered list. It is related to the quicksort sorting algorithm. Like quicksort, it is based on the divide and conquer paradigm. The average time complexity is O(n), but the worst-case time complexity is O(n^2). If a guaranteed O(n) solution is required, algorithms like Median of Medians can be used, though they are more complex to implement.